Grain size effects on Ni/Al nanolaminate combustion
- PDF / 1,262,249 Bytes
- 10 Pages / 584.957 x 782.986 pts Page_size
- 23 Downloads / 233 Views
FOCUS ISSUE
INTRINSIC AND EXTRINSIC SIZE EFFECTS IN MATERIALS
Grain size effects on Ni/Al nanolaminate combustion Brandon Witbeck1 Douglas E. Spearot2,a) 1
Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, Florida 32611, USA; and Air Force Research Lab, Munitions Directorate, Eglin AFB, Florida 32542, USA 2 Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, Florida 32611, USA a) Address all correspondence to this author. e-mail: dspearot@ufl.edu Received: 4 December 2018; accepted: 29 January 2019
Reactions in Ni/Al nanolaminates exhibit high combustion temperatures and wave speeds that are customizable through changes to nanostructure. Nanolaminates fabricated via vapor deposition exhibit columnar grains with average diameters on the order of the individual layer thickness; yet, their role on nanolaminate combustion has not been previously investigated. The current work uses molecular dynamics simulations to elucidate the effect of grain size on reaction rates and combustion temperatures in Ni/Al nanolaminates. Decreasing grain size is shown to increase reaction rates as well as increase peak temperatures consistent with the excess enthalpy of smaller grain sizes. Additionally, grain boundaries provide heterogenous nucleation sites for the diffusion-restricting B2–NiAl phase. Focusing on Ni diffusion into liquid Al, an effective diffusion coefficient is computed as a function of grain size, which may be used in thermodynamic models for this stage of the reaction.
Introduction Reactions in Ni/Al nanolaminates are a topic of scientific interest due to the magnitude of exothermic output and the ability to modify combustion characteristics. For example, calculations predict adiabatic combustion temperatures up to 1912 K, and experiments show ignition at temperatures below 500 K [1]. Such combustion characteristics have found industrial application in soldering microelectronic components [2] and have been proposed for military use as microinitiators in ordnance systems [3, 4]. The combustion characteristics of Ni/Al nanolaminates can be modified through changes to initial nanostructure. The most fundamental structural aspect is the bilayer thickness, defined as the combined length of one Ni and one Al layer. Reducing this bilayer thickness, down to ;19 nm for samples with limited annealing and interface premixing, results in increasing combustion temperatures and wave speeds [5, 6]. Further reductions in the bilayer thickness result in decreasing wave speeds, attributed to an increased volume fraction of premixing at the Ni/Al interface, which reduces stored energy available for combustion [5]. Likewise, several authors have shown that increasing this premixed region leads to decreasing combustion wave velocity across all bilayer thicknesses studied [7, 8, 9].
ª Materials Research Society 2019
Increasing misfit strain along the Ni/Al interface has also been shown to reduce the ignition temperature [10]. The Ni/Al nanolaminate reaction has been divi
Data Loading...
